Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
131 PM EDT Sun Aug 18 2019

Valid Aug 18/1200 UTC thru Aug 22/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z NAM/GFS Evaluation...Including Preliminary Preferences and
Confidence
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...Overall Pattern Across the CONUS...
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Preference: 12Z NAM, 12Z GFS, 00Z ECMWF blend
    less weight on 12Z GFS across FL Panhandle/Gulf Coast
Confidence: Average

The main component of mid-upper level ridging in place at the
start of the period near the Rio Grande will translate westward to
just off of the Southwest coast by Wednesday afternoon. A small
portion of the ridge will remain over the south-central U.S.
Meanwhile, across the northern tier of the U.S., the flow is
quasi-amplified with a closed low in south-central Canada which
will track east and elongate east-west before reamplifying
southward toward the Great Lakes late Wednesday. Back to the west,
an anomalous upper trough, forecast to be offshore of the Pacific
Northwest on Tuesday, will weaken slightly as it moves into
British Columbia and the Pacific Northwest Wednesday.

There are some minor timing differences with the upper trough in
the vicinity of the Great Lakes, but larger differences concerning
the handling of a mid-level vorticity max currently over the
eastern Florida Panhandle. The 12Z GFS is the quickest to take the
related 850-700 mb low northeastward over the next 2-3 days while
the 00Z ECMWF/CMC hang the feature back. The 12Z NAM is somewhere
in the middle while the 00Z UKMET is rather strong with the
feature compared to the remaining available guidance. A middle
ground between the 12Z NAM, 12Z GFS and 00Z ECMWF is preferred,
but with less weight on the 12Z GFS given a lack of supporting
guidance.

Across the Pacific Northwest, only the 00Z CMC stands out
significantly with a faster trough progression, which shows up in
the ensemble spaghetti guidance as well. The 12Z NAM is slightly
more amplified than the consensus across the Northwest but impacts
to the U.S. given these differences appear minor. Blending the 12Z
NAM/GFS and 00Z ECMWF will work well for this part of the nation.

Model trends at www.wpc.noaa.gov/html/model2.html
500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml

Otto